σ-Electrons Responsible for Cooperativity and Ring Equalization in Hydrogen-Bonded Supramolecular Polymers.

Invited for this month's cover are collaborators from the TheoCheM group of the Vrije Universiteit Amsterdam and the University of Perugia. The cover picture shows a σ-electron traveling through a hydrogen-bonded squaramide linear chain. The charge transfer within the σ-electronic system is the cause for the cooperativity in the investigated urea, deltamide, and squaramide polymers. More information can be found in the Full Paper by Célia Fonseca Guerra, and co-workers.

σ-Electrons Responsible for Cooperativity and Ring Equalization in Hydrogen-Bonded Supramolecular Polymers.

We have quantum chemically analyzed the cooperative effects and structural deformations of hydrogen-bonded urea, deltamide, and squaramide linear chains using dispersion-corrected density functional theory at BLYP-D3(BJ)/TZ2P level of theory. Our purpose is twofold: (i) reveal the bonding mechanism of the studied systems that lead to their self-assembly in linear chains; and (ii) rationalize the C-C bond equalization in the ring moieties of deltamide and squaramide upon polymerization. Our energy decomposition and Kohn-Sham molecular orbital analyses reveal cooperativity in all studied systems, stemming from the charge separation within the σ-electronic system by charge transfer from the carbonyl oxygen lone pair donor orbital of one monomer towards the σ* N-H antibonding acceptor orbital of the neighboring monomer. This key orbital interaction causes the C=O bonds to elongate, which, in turn, results in the contraction of the adjacent C-C single bonds that, ultimately, makes the ring moieties of deltamide and squaramide to become more regular. Notably, the π-electron delocalization plays a much smaller role in the total interaction between the monomers in the chain.

Chemical Bond Mechanism for Helium Revealed by Electronic Excitation.

Helium chemistry is notoriously very impervious. It is therefore certainly no surprise that, for example, beryllium and helium atoms, in their ground state, do not bind. Full configuration-interaction calculations show that the same turns out to be true, save for a long-range shallow attraction, for the Be+ + He system. However, quite astonishingly, when one electron is re-added to Be+ in an excited 2pπ or 3s orbital (Be 1P or 1S), a bound adduct with He is formed, at an interatomic separation as short as 1.5 Å. Understanding why this happens reveals an unsuspected chemical mechanism that stabilizes helium compounds at the molecular level.

Insight into the halogen-bond nature of noble gas-chlorine systems by molecular beam scattering experiments, ab initio calculations and charge displacement analysis.

We have carried out molecular-beam scattering experiments and high-level ab initio investigations on the potential energy surfaces of a series of noble-gas-Cl2 adducts. This effort has permitted the construction of a simple, reliable and easily generalizable analytical model potential formulation, which is based on a few physically meaningful parameters of the interacting partners and transparently shows the origin, strength, and stereospecificity of the various interaction components. The results demonstrate quantitatively beyond doubt that the interaction between a noble-gas (Ng) atom - even He - and Cl2 in a collinear configuration is characterized by weak halogen bond (XB) formation, accompanied by charge transfer (CT) from the Ng to chlorine. This characteristic, which stabilizes the adduct, rapidly disappears on going towards the T-shaped configuration, dominated by pure van der Waals (vdW) forces. Similarly, a pure vdW interaction takes place - with no CT component in any configuration - if Cl2 is present in the lowest πg* → σu* excited state, because the change in electron density that accompanies the excitation eliminates the Cl2 polar flattening and σ hole, making the XB interaction inaccessible.

Selective Emergence of the Halogen Bond in Ground and Excited States of Noble-Gas-Chlorine Systems.

Molecular-beam scattering experiments and theoretical calculations prove the nature, strength, and selectivity of the halogen bonds (XB) in the interaction of halogen molecules with the series of noble gas (Ng) atoms. The XB, accompanied by charge transfer from the Ng to the halogen, is shown to take place in, and measurably stabilize, the collinear conformation of the adducts, which thus becomes (in contrast to what happens for other Ng-molecule systems) approximately as bound as the T-shaped form. It is also shown how and why XB is inhibited when the halogen molecule is in the 3 Πu excited state. A general potential formulation fitting the experimental observables, based on few physically essential parameters, is proposed to describe the interaction accurately and is validated by ab initio computations.

The role of the halogen bond in iodothyronine deiodinase: Dependence on chalcogen substitution in naphthyl-based mimetics.

The effects on the activity of thyroxine (T4) due to the chalcogen replacement in a series of peri-substituted naphthalenes mimicking the catalytic function of deiodinase enzymes are computationally examined using density functional theory. In particular, T4 inner-ring deiodination pathways assisted by naphthyl-based models bearing two tellurols and a tellurol-thiol pair in peri-position are explored and compared with the analogous energy profiles for the naphthalene mimic having two selenols. The presence of a halogen bond (XB) in the intermediate formed in the first step and involved in the rate-determining step of the reaction is assumed to facilitate the process increasing the rate of the reaction. The rate-determining step calculated energy barrier heights allow rationalizing the experimentally observed superior catalytic activity of tellurium containing mimics. Charge displacement analysis is used to ascertain the presence and the role of the electron density charge transfer occurring in the rate-determining step of the reaction, suggesting the incipient formation or presence of a XB interaction. © 2019 Wiley Periodicals, Inc.

Modelling Charge Transfer in Weak Chemical Bonds: Insights from the Chemistry of Helium.

We studied the nature of the interaction of the weakly bound Be-He adduct by means of an integrated theoretical approach based on high-level quantum chemical calculations for the characterization of the potential energy surfaces and charge displaced upon adduct formation, together with the development of a semi-empirical analytical formulation of the interaction potential. Our results show that Be is able to form a stable adduct with He when the Be(1 D) (1s2 2s2 →1s2 2s0 2p2 ) excited state is involved, with a binding energy of as much as 10.2 kcal/mol, an astonishingly large value for He in neutral systems. The analysis of the leading interaction components in the Be*-He adduct proves the relevance of the charge transfer to the overall stability, which contributes to decreasing the intermolecular distance, thus strengthening the induction-energy component.

Complexation of Al3+ and Ni2+ by l-Ascorbic Acid: An Experimental and Theoretical Investigation.

Despite the potential versatility of Vitamin C as a ligand, only for few metal complexes does full characterization exist. Vitamin C metal complexes, indeed, are difficult to study experimentally because the ligand has multiple protonation and oxidation states, and the metal-ligand complexes are in general not as tight as one might expect and frequently resistant to crystallographic characterization. In most cases, coordination via one of the hypothesized modes is invoked, characterized by monodentate or bidentate coordination via the most acidic oxygen atoms, with the ligand singly or doubly deprotonated. In this study the ability of l-ascorbic acid to form complexes with Al3+ and Ni2+ ions under physiological conditions was investigated by using a combination of potentiometric measurements, 1H NMR spectroscopy, and DFT computations in order to recognize the structural properties of the resulting complexes in aqueous solution. The comparison between the values of the free energies of complexation obtained by using DFT quantum chemical calculations and estimated from experimental stability constants according to the mass action law and by considering the involved equilibria allows to select structure and preferred coordination modes of formed complexes. The protonation constant of the free ligand was also determined using potentiometric data and its reproduction by using computational approaches was critically commented.

Helium Accepts Back-Donation In Highly Polar Complexes: New Insights into the Weak Chemical Bond.

We studied the puzzling stability and short distances predicted by theory for helium adducts with some highly polar molecules, such as BeO or AuF. On the basis of high-level quantum-chemical calculations, we carried out a detailed analysis of the charge displacement occurring upon adduct formation. For the first time we have unambiguously ascertained that helium is able not only to donate electron density, but also, unexpectedly, to accept electron density in the formation of weakly bound adducts with highly polar substrates. The presence of a large dipole moment induces a large electric field at He, which lowers its 2p orbital energy and enables receipt of π electron density. These findings offer unprecedented important clues toward the design and synthesis of stable helium compounds.